Glossary of sleep terms

Do you know what sleep apnea is? Or narcolepsy? If not, this glossary of sleep terms will be helpful. Here we cover the definitions and symptoms of various sleep disorders. This blog post includes a list of resources for those looking to learn more about sleep or find out how they can improve their own sleeping habits.

Actigraphy: an unobtrusive way of monitoring sleep, by wearing a watch-like device on the wrist.

Alpha Intrusion: a fragment of alpha wave sleep that intrudes into a person’s beta or wakefulness. It is proven to be connected with a higher blood pressure [1]Alpha Intrusion during Rapid Eye Movement (REM) Sleep Increases Blood Pressure

Arousal: is a sudden change in the level of consciousness, as from sleep to wakefulness

Catnap: a sleep disorder where an individual falls asleep for up to 15 minutes, usually in the afternoon.

Cheyne Stokes Respiration: a breathing pattern in which a person’s breath is deeper and more frequent at the end of an exhalation than during inhalation.[2]Cheyne Stokes Respirations

Chronotherapy: a process of gradually delaying sleep by typically half an hour a day for one to two weeks.

Circadian rhythm sleep disorder: A sleep disorder in which a person has trouble sleeping because their body’s “clock” is out of sync with the desired time. Basically, it is a person sleeping during the day and awake at night.

Circadian Rhythms: Biological rhythms that cycle in approximate 24 hour periods and are driven by light exposure (natural or artificial) and other environmental cues

Conditioned Insomnia: a kind of insomnia that is seen in people who have a sleep disorder and has been around for five years or more.

Creatine Kinase MB (CKMB): an enzyme released from skeletal muscles during the sleep cycle. It is a marker for cardiac injury. CKMB levels are proven to be correlated with acute myocardial infarction in chest pain. [3]Serial creatine kinase-MB results are a sensitive indicator of acute myocardial infarction in chest pain patients with nondiagnostic electrocardiograms: the second Emergency Medicine Cardiac Research … Continue reading

Daytime Drowsiness: A state of being tired and sleepy during the day that feels like they need more sleep than usual. This can be caused by having insufficient sleep, not getting enough deep sleep, or excessive stress.

Daytime Sleep Latency: the period between going to bed at night and falling asleep in daytime hours. It measures how long it takes for someone to fall asleep during the day.

Drowsiness: is a state of mind or body characterized by lack of alertness, weariness from exertion; sleepiness. However, it is not the phase of sleep.

Heart Rate: the rate at which the human heart is beating

Hypersomnia: sleeping more than twelve hours during the night

Hypnagogic Hallucinations: A hallucination that occurs in the hypnogogic state, which is when we are half awake and still half asleep (right before we fall into sleep). It can be a fear of dying or other intense thoughts. It can be related with REM sleep[4]What Is the Link Between Hallucinations, Dreams, and Hypnagogic–Hypnopompic Experiences?

Hypnogram: A sleep stage chart that illustrates waxing and waning of brain waves, eye movements, breathing patterns, and muscle activity during sleep stages.

Insomnia: A medical disorder in which a person has difficulty falling asleep, staying asleep (due to frequent awakenings or early morning awakening), or getting restful sleep. Insomnia can result from insufficient sleep quantity and/or quality or poor sleeping habits such as using electronics before going to bed.

Insomnia: difficulty falling asleep or staying asleep due to anxiety about not sleeping well enough.

Insufficient sleep: a situation when a person does not receive an adequate amount of sleep. This may result in excessive daytime sleepiness, mood changes, or difficulty concentrating.

Jet lag: a condition that occurs when your sleep schedule is suddenly disrupted by travel across several time zones.

Light sleep: A sleep stage characterized by a low amount of brain activity, theta waves, and rapid eye movements.

Light therapy: a specialized treatment of sleep disorders in which a person is exposed to light during the day. This improves melatonin production and helps regulate a person’s circadian rhythm.

Limit-Setting Sleep Disorder: is a sleep disorder where the individual has trouble sleeping because they are afraid of things that might happen during their dreams.

Lucid dream: it is a dream in which the dreamer is aware that he or she is dreaming. A person is capable of controlling their actions in a lucid dream and can tell when they’re awake.

Melatonin: a hormone produced in the pineal gland. Melatonin is released from the brain and produced by a process of oxidative decarboxylation, which converts serotonin into melatonin. Its role in the human body is to regulate the sleep cycle, via its natural hypnotic effects.

Microsleep: a short period of sleep that lasts for under 30 seconds and occurs in a state between wakefulness and sleep. These periods are so brief the person is not aware they have fallen asleep, but there can be significant consequences such as go off the road or crashing into another car on autopilot because of microsleep episodes.

Midnight snacker: Someone who eats just before bedtime with the intention of sleeping better or maintaining their weight, but usually wind up eating more than they actually need and getting into bad habits such as overeating late at night.

Mixed Sleep Apnea: This is a sleep disorder where an individual has both Obstructive and Central Sleep Apnea. It is not considered to be a disease[5]Complex Sleep Apnea: It Really Is a Disease

Monophasic sleep: the sleep episode that consists of only one sleep stage.

Movement Arousal: is a sleep disorder where the individual is unable to fall asleep because of physical movements.

Multiple sleep latency tests (MSLT): a test that is used to determine whether a person has sleep apnea.

Napping: Taking an afternoon nap lasting anywhere from fifteen to thirty minutes. This has been shown to improve mood, productivity, and alertness for the next few hours.

Narcolepsy: A chronic neurological disorder characterized by overwhelming daytime drowsiness, uncontrollable attacks of refreshing nighttime sleep, abnormal REM patterns in the brain, cataplexy (sudden loss of muscle tone), hypnagogic hallucinations (waking dreams, etc).

Night owl: A person who prefers to go to bed late and wake up late. They tend to be more intelligent and creative, but are also at higher risk for health problems such as heart disease due to their sleep habits.

Nightmare: a dream that involves intense fear and unpleasant feelings. They are often caused by fear-provoking thoughts in the dreamer’s mind during unconsciousness or lack of consciousness.

Nighttime Sleep Latency: the period between going to bed at night and falling asleep in nighttime hours. It measures how long it takes for someone to fall asleep during the night.

Nocturia: Frequent urination during the night that is caused by the production of urine beyond the capacity of the bladder or diseases such as diabetes.

Nodding off: a person’s head begins to tilt forward and will sometimes shake back and forth as the person falls asleep.

Non-Rapid Eye Movement Sleep: is a phase of sleep that is characterized by low-voltage fast activity on an electroencephalogram and a relatively low frequency of eye movements.

Optimal Sleep: the amount of sleep required for an individual to function optimally.

Orexin: A neuropeptide that regulates wakefulness and appetite via interaction with the hypothalamus. Orexin is proven to play an important role in human sleep.[6]New Approaches for the Study of Orexin Function

Parasomnia: An abnormality in the person’s behavior that occurs during sleep (other than just snoring). The word parasomnia comes from Greek pará “beside” + somnían “to produce sound while asleep.”

Paroxysmal Nocturnal Dyspnea (PND): is a sleep disorder that is characterized by difficulty breathing in the middle of the night.

Partial sleep deprivation: occurs when sleep duration is above 1 minute but less than 8 hours and is often the result of a sleep disorder such as narcolepsy.

Perceptual Disengagement: is the point where sleep starts to take place and awareness of your surroundings is diminished.

Periodic Limb Movement Disorder (PLMD): is a sleep disorder that causes the individual to move their limbs, usually occurring every 20-40 seconds.

The phase of sleep: is a distinctive state that can be observed in sleeping subjects on polysomnography recordings, characterized by rapid eye movements (REM) or other markers.

Polyphasic sleep: A polyphasic sleeper sleeps for multiple brief periods throughout one 24 hour period as opposed to sleeping once.

Polysomnography: a sleep diagnostic test. It involves the recording of several physiological parameters, usually including brain waves (electroencephalogram), eye movements, or electrooculography) and muscle tone throughout the sleep cycle.

Positive airway pressure (PAP) device: is a device specifically designed to provide therapy in the form of a respiratory pressure, that is delivered to create increased airway patency and decrease resistance during sleep.

Post-Prandial Drowsiness: a desire to sleep after eating a large meal. This can be explained by the fact that when a person eats, blood flow is directed away from the brain and into the stomach.

Premature Morning Awakening: a sleep disorder where an individual wakes up before the end of a sleep cycle. There can be several reasons for it such as stress, medicines, or other causes.

Progressive Muscle Relaxation (PMR): a method of reducing physical tension and anxiety that involves tensing then relaxing muscle groups in a systematic order.

REM Motor Atonia: a sleep disorder where a person’s arms or legs are paralyzed while dreaming.

REM Onset: the stage in a sleep cycle where dream activity begins to occur.

REM Rebound: a sleep disorder where an individual feels more sleepy after waking up from REM sleep.

REM Sleep (Stage IV Sleep): is a phase of sleep that has a higher level of brain activity and quicker eye movements than in the other sleep stages. REM is believed to be important for memory, learning, and creativity.

REM Sleep Episode: an episode of REM sleep, which is a stage in the sleep cycle where dreaming occurs.

REM: Rapid Eye Movement, characterized by the presence of high-voltage slow waves with frequent bursts that appear as a rapid movement in the eyes; it is usually associated with dreaming.

Restless Legs Syndrome (RLS): a chronic disorder where an individual has to move their legs because of a tingling sensation.

Restlessness: in terms of quality of sleep, this refers to when an individual shifts positions often or tosses and turns.

Screen time: the total amount of time a person spends looking at a screen daily. Both computer, reader, and smartphone screen times should be added.

Sedative: a substance that induces sleep.

Sleep aid: a specialized substance, such as sleep medication, that is intended to improve sleep duration and quality

Sleep Apnea: a common disorder in which people have one or more pauses in breathing or periods of shallow breathing during sleep.

Sleep architecture: the cyclical pattern of the sleep stages that includes NREM and REM sleep

Sleep bruxism: grinding the teeth during the night sleep. This can be harmful because it can lead to wear on the teeth, and tooth damage

Sleep cycle: A complete sequence of stages of NREM followed by REM sleep over the course of one night’s sleep; this pattern repeats itself several times

Sleep debt: sleep deprivation that can accumulate over time, to the point where it becomes dangerous.

Sleep definition: The natural periodic suspension of consciousness during which restorative processes take place in the body and mind.

Sleep deprivation: when one has been deprived of sleep for a long enough time that it impairs their ability to function normally. It affects the body by suppressing immune responses, altering moods and behavior patterns as well as impairing memory.

Sleep disorder: a condition that causes either abnormal patterns or excessive quantity of nighttime sleep.

Sleep duration: the total amount of time spent asleep

Sleep efficiency: the proportion of time spent in bed that is actually sleeping.

Sleep environment: a set of bedding furniture, accessories, and a room where a person sleeps
Sleep fragmentation: interruptions of sleep that occur due to changes in sleep position, snoring, or other noise that cuts off one’s breathing, disturbing sounds, or lighting.

Sleep hygiene: a set of habits in order to create a chain of sleep that is good for one’s health.

Sleep Hyperhydrosis: a sleep disorder in which a person sweats excessively while sleeping.

Sleep inertia: an effect of feeling groggy or out of it for a few hours after waking up

Sleep latency: the amount of time required to fall asleep.

Sleep Paralysis: a sleep disorder that prevents an individual from moving about in any way and is often associated with the feeling of a heavy weight on their chest.

Sleep patterns: common regularities of sleep that are found in people.

Sleep quality: a characteristic showing the level and depth of sleep

Sleep regression: a tendency of decrease in sleep quality and duration following a period of improving sleep

Sleep Talking: a sleep disorder caused by the individual’s complex and sometimes incoherent speech during sleep.

Sleep Terrors: a type of parasomnia that causes an individual to wake up suddenly in a panic and is usually accompanied by screaming, though the individual does not remember it.

Sleep-related breathing disorder: a sleep disturbance characterized by difficulties in controlling one’s breath while asleep.

Sleep-wake cycle disorder: a sleep disturbance characterized by persistent problems with the timing of one’s sleep and wakefulness.

Sleep-wake cycle: the natural pattern that one’s body follows in its daily periods of activity and rest.

Sleeping person: a person that is sleeping.

Sleeping position: a body position that a person sleeps in

Sleepwalking: walking and performing other activities while in the state of sleep – often a sign of deep sleep deprivation

Slow-wave sleep (SWS): is deep dreamless sleep that recurs in cycles with increasing intensity. SWS dominates early in the night and accounts for about 80% of total sleep time. It’s also called Stage III or delta sleep because it has a slower electrical activity than the other types of sleep.

Snoring: an act of producing sound by creating a partial blockage of the upper airway, which is most common in overweight people.

Somnolence: a state of feeling drowsy or sleepy

Stage I (NREM) or N-Sleep: Stage One or Non-Rapid Eye Movements sleep is characterized by light sleep, thoughts slowing down and one feels very relaxed.

Stage II or N-REM: Stage Two or Non-Rapid Eye Movements sleep: is characterized by more intense sleep compared to stage one because the brain waves are synchronized with each other.

Stage III sleep: Stage Three or Slow Wave Sleep: is characterized by sleep that involves long periods of deep sleep.

Stimulant: any substance with an invigorating effect like caffeine; substances that promote wakefulness and alertness (typically drugs)
Subwakefulness Syndrome: a disorder that prevents a person from fully awakening.

The suprachiasmatic nucleus (SCN): is a part of the brain located in the hypothalamus that regulates when we sleep and wake.

Time in bed: the amount of time dwelling in bed before sleep onset, which can be estimated by adding up the total number of hours spent sleeping to the total number of minutes awake after sleep onset.

Total sleep time (TST): is the total duration of sleep that a person gets in one night. It can be used to assess sleep quality. It is a sum of REM + NREM sleep during a single sleep episode.

Variable sleep-wake syndrome: A condition marked by irregular sleeping hours, which may be caused or aggravated by shift work (i.e., changing shifts every week).

Variety: refers to how different a person’s sleep schedule is from one night to another; variability or inconsistency within a given day.

Vivid dream: a type of dream that is typically more memorable and emotionally intense than other types of dreams.

Wake-up threshold: the lowest amount of sleep loss or time spent awake that will allow a person to fall asleep again; also called sleep debt, it reflects how many minutes are needed for an individual’s brain waves to return into their natural pattern.

Wakefulness: is a state of mind and body being awake and active.

White Noise: a sound produced by machines or software which emits sound waves at frequencies that are similar to those experienced during deep sleep. These sounds help promote relaxation and a peaceful mind.

Zeitgeber: a factor that contributes to a person’s sleep timing and duration, such as daylight or the work schedule.

How the sleep terminology can help you:

In our articles, we often consult with a huge range of doctors and sleep experts. This glossary will help you to better understand our articles and what they’re discussing, find out how you can improve your quality of sleep, resolve the sleep issues and pick the right solutions exactly for your situation.

Do you have any other terms suggestions?

Leave a comment below, or find us on social media to share any sleep terms you would like to see in our glossary. We are open to extending this glossary to fully cover sleep-related terminology.

The most important sleep terms

Sure, this vocabulary contains quite a big amount of terms. Here are the most important sleep terms that are the most important:

  • Insomnia
  • Sleep duration
  • Sleep quality
  • Sleep phases
  • Circadian rhythms
  • Melatonin
  • Sleep deprivation

In a word

We hope that this list will help you to understand the terms that are related to sleep. If you want to know more about this topic, we recommend reading all our sleep-related articles that will provide a deep overview of everything there is to know about it and give some great advice for how to improve your sleep quality, duration, and phases.


1Alpha Intrusion during Rapid Eye Movement (REM) Sleep Increases Blood Pressure
2Cheyne Stokes Respirations
3Serial creatine kinase-MB results are a sensitive indicator of acute myocardial infarction in chest pain patients with nondiagnostic electrocardiograms: the second Emergency Medicine Cardiac Research Group Study
4What Is the Link Between Hallucinations, Dreams, and Hypnagogic–Hypnopompic Experiences?
5Complex Sleep Apnea: It Really Is a Disease
6New Approaches for the Study of Orexin Function

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